1. Field of the Invention
The present invention relates to a filtration process using a filter column comprising a hollow fiber membrane module(s), which is used to treat condensate water obtained in a nuclear or thermoelectric power plant as well as waste water or the like.
2Related Art
In general, a hollow fiber membrane filter column comprises a number of hollow fiber membrane modules including a large number of hollow fibers bundled together therein to serve as porous hollow fiber membranes. The hollow fiber membrane modules are juxtaposed with each other generally in a state of being hung from a module support laterally provided in the filter column. Conventional filtration processes using such a hollow fiber membrane filter column basically comprise a filtration step, a scrubbing step and a drain step. Specifically, in the filtration step, raw water is fed into a lower compartment partitioned with the module support to allow the raw water to permeate through the pores of the.hollow fiber membranes from the outsides of the hollow fibers to the insides thereof. Fine particles in the raw water are trapped on the outer surfaces of the hollow fibers, while filtrate obtained on the insides of the hollow fibers is gathered in an upper compartment partitioned with the module support and is then drained out of the filter column.
When the differential pressure, or pressure drop, across the filter column is increased to some level through the filtration step, bubbles are charged into water around the outer surfaces of the hollow fibers dipped in water to cause agitation of the water with which the hollow fiber membranes are vibrated to exfoliate the fine particles trapped on the outer surfaces of the hollow fibers. This step, which is called a scrubbing step, is followed by the drain step of discharging from the lower compartment the waste water resulted from the scrubbing and containing the exfoliated fine particles. Thus, the operation of the filter column involves the filtration step, the scrubbing step and the drain step.
Additionally stated, the backwash step of inversely flowing washing water from the insides of the hollow fibers to the outsides thereof may sometimes be taken before, during or after the scrubbing step.
As means for charging bubbles such as air bubbles around the outer surfaces of the hollow fibers, there can be mentioned release of a gas such as air on the lower side of the hollow fiber membrane modules to charge air bubbles into the insides of the hollow fiber membrane modules, and injection of a compressed gas such as compressed air into the central bores of the hollow fibers to release the gas through the pores of the hollow fiber membranes to form bubbles around the outer surfaces of the hollow fibers. The former is employed in most cases where use is madc of hollow fiber membranes having such fine pores as to hardly allow a gas to pass therethrough, while the latter is employed in most cases where use is made of hollow fiber membranes having such fine pores as to allow a gas to easily pass therethrough.
Thus, the filter column using hollow fiber membranes is basically operated by repeating the filtration step, the scrubbing step and the drain step. In the repetition of these steps, sufficient care is needed to ensure that continuation of filtration is not obstructed due to irreparable accumulation of fine particles on the outer surfaces of the hollow fibers which raises the differential pressure across the filter column.
Therefore, investigations, tests and developments have heretofore been made of the structure of hollow fiber membrane modules, the structure of filter columns, methods of washing hollow fiber membranes by scrubbing in particular, etc. with an eye to preventing fine particles from accumulating while trapped on the outer surfaces of hollow fibers.
The authors of the present invention have made efforts as well to develop a process for effectively washing hollow fiber membranes in hollow fiber membrane modules disposed in a filter column. They have found out, however, that, in some cases of treating such raw water containing fine iron oxide particles as condensate water (primary cooling water) obtained, for example, in a BWR nuclear power plant, the differential pressure across a hollow fiber membrane module, increased through the filtration step, is not reverted to the original one even when the module is subjected to the aforementioned scrubbing and backwash steps or even when those scrubbing and backwash steps are further followed by subjecting the hollow fiber membrane module to the step of washing hollow fiber membranes with an acid to dissolve and remove therefrom the iron oxides adhering to the outer surfaces of the hollow fiber membranes.
With a view to looking into the causes of the foregoing phenomenon, the authors of the present invention have made various investigations and experiments, as a result of which the following have been elucidated:
(1) A cause for non-reversion of the differential pressure is lowering of the water permeability of the hollow fiber membranes.
(2) This lowering of water permeability of hollow fiber membranes is not due to compaction and/or crush of the hollow fiber membrances owing to the pressure differential between the outside and inside thereof, but because the outer surface portions of the hollow fibers are, so to speak, roughened such that most pores originally present are occluded to considerably decrease the number of open pores all across the hollow fiber membranes.
(3) The roughened state of the outer surface portions of the hollow fibers does not change even when the hollow fibers are washed with a chemical such as an acid, an oxidizing agent or a reducing agent.
(4) The non-reversion of the differential pressure is not a result of deterioration of such physical properties of the hollow fibers as manifested in terms of lowerings of their mechanical strengths such as tensile strength, tensile elongation and crushing strength.
As a result of further investigations and experiments, the following have further been elucidated:
(5) The outer surface portions of the hollow fibers are roughened through collision thereagainst of fine particles such as iron oxide particles.
(6) The collision of the fine particles against the outer surfaces of the hollow fibers occurs when the fine particles exist between the hollow fibers being vibrated.
(7) The above-mentioned collision is liable to occur most frequently in the step of scrubbing the hollow fiber membranes.
(8) The roughening of the outer surface portions of the hollow fibers are greatly affected by the temperature of water in contact therewith in such a way that the outer surface portions of the hollow fibers are liable to be roughened more as the temperature of water is higher.
The present invention has been made based on these findings.
Hence, an object of the present invention is to provide a filtration process using at least one hollow fiber membrane module including a large number of hollow fiber membranes and generally disposed in a filter column, according to which the roughening of the outer surface portions (hereinafter often referred to in brief as the outer surfaces.) of the hollow fiber membranes can be greatly suppressed without lowering the exfoliative ability of fine particles trapped on the outer surfaces of the hollow fibers.